Pressure transducers provide pressure measurements for many applications from scientific experiments to control of industrial systems and machines. In these applications, pressure changes need to be measured and converted into electrical signals representing the pressure changes. In some instances, a differential pressure transducer is used to detect pressure differences between two applied bias pressures. In many cases the media being measured is corrosive or conductive and requires a "wet/wet" differential pressure transducer. This type of transducer has two pressure ports which may be exposed to the process media without damage to the sensitive pressure measuring apparatus. The common technologies for measuring this phenomenon are capacitance or reluctance devices requiring complex alternating current electronics and multiple passive isolation diaphragms. Bonded foil strain gage devices are also commonly used with passive isolation diaphragms, but are not practical for the low differential pressure ranges common in these applications.
U.S. Pat. No. 4,462,259 describes a manometer type differential pressure gauge using a column of ferrofluid to act as the dielectric medium between two plates on either side of the column, forming a capacitor. The length of the ferrofluid in the column is dependent upon an applied pressure differential, so that the capacitance in the capacitor thus formed is dependent upon the applied pressure.
A magnetic field is used to act on the magnetic fluid to bias the slug of fluid toward the reference position. The magnetic field creates an additional force which adds to the gravitational forces acting on the magnetic field, a given column length of the magnetic liquid can represent a pressure differential which is larger than that obtained absent the application of the magnetic field.
This design, however, can only be used with clean gases, as the ferrofluid is exposed to the process medium. If high over-pressure situations develop, the ferrofluid could be expelled from the manometer tube. Devices made according to this design also require complex AC electronics to convert the differential column height of the ferrofluid to a useable electric signal.
U.S. Pat. No. 4,788,867 describes a pressure transducer having a permanent magnetic "piston," suspended in a tube by a body of magnetic fluid. Pressure differentials applied to the tube cause the piston to move within the tube. A linear variable differential transformer (LVDT) measure the displacement of the piston. A second pair of cavities separate the magnetic fluid from the process media and provide over-pressure protection. The ferrofluid property is thus used to float a ferromagnetic material, acting as a frictionless seal for the magnetic piston. This moving piston has substantial mass and is therefore sensitive to gravitational effects.